Wastewater from both municipal sewage systems and from industrial waste product exhausting systems is usually collected in large ponds, ditches, or basins that are referred to as wastewater ponds. Such ponds may be a few to several feet deep and may cover quite a number of acres of surface area. The wastewater usually includes large amounts of organic and inorganic waste material that, if left untreated, creates severe odors and can generates toxic products.
Moreover, EPA has published dissolved oxygen (DO) criteria for liquid, such as fresh, salt and brackish water, and wastewater, sewage and industrial wastewater discharges into the same bodies of water to protect organisms and their uses from the adverse effects of low DO conditions. The Agency developed these criteria because hypoxia (low dissolved oxygen) is a significant problem for lakes, streams, rivers, and coastal waters that receive a lot of runoff that contain nutrients (for example, nitrogen and phosphorous and other oxygen-demanding biological wastes). Excessive nutrients in aquatic systems stimulate algae growth, which in turn uses up the oxygen needed to maintain healthy fish, shellfish, and other aquatic life populations.
EPA's Environmental Monitoring and Assessment Program (EMAP) for lakes, streams, rivers, and coastal waters has shown areas exposed to some dissolved oxygen concentrations of less than 5 mg/L. Long periods of DO below 5 mg/L can harm larva life stages for many fish, shellfish, and other aquatic life populations.
The EPA's dissolved oxygen criteria apply to both continuous and cyclic low DO conditions. If the DO conditions are always above the chronic criterion for growth (4.8 mg/L), the aquatic life at that location should not be harmed. If the DO conditions at a site are below the juvenile/adult survival criterion (2.3 mg/L), there is not enough DO to protect aquatic life.
Under the Clean Water Act (CWA), states, territories, and tribes must adopt water quality criteria to protect designated uses. The EPA has promulgated regulations to implement this requirement including levels of DO (see 40 CFR 131).
The most common method of wastewater treatment uses an activated sludge process. This process involves three major steps. The primary treatment stage consists of a simple separation between dense sludge, which is sent to an incinerator or land fill, and the remaining effluent liquid sludge then undergoes secondary treatment. Secondary treatment is where the biochemical consumption of organic material takes place. The microorganisms present in the liquid sludge feast on the biomass in the wastewater pond. Extensive aeration is needed for the bacteria to consume the organic wastes.
The third phase of treatment can be simple or extensive depending upon the extent of pollution and the requirements for water purity. Its purpose is to remove inorganic pollutants as well as any organic mass not removed by the primary and secondary stages. Lastly, the treated water is discharged back into the environment. This discharge must meet federal, state, county and city government standards for discharged water, such as minimum dissolved oxygen levels deemed necessary to accommodate marine life, before such wastewater can be discharged into a river or stream.
The activated sludge process is a biochemical process in which aerobic bacteria consume the organic pollutants in wastewater. Because the bacteria are aerobic, their efficiency of consumption is very dependent upon the amount of available oxygen dissolved in the liquid sludge. In the wastewater treatment process, aeration introduces air into a liquid, providing an aerobic environment for microbial degradation of organic matter. The purpose of aeration is two-fold: to supply the required oxygen to the metabolizing microorganisms and to provide mixing so that the microorganisms come into intimate contact with the dissolved and suspended organic matter.
Various aeration approaches have been used; the two most common aeration systems are subsurface and mechanical. In subsurface aeration systems, air or oxygen is pumped below the surface to a diffuser or other device submerged in the wastewater. Fine pore diffusion is a subsurface form of aeration in which air is introduced in the form of very small bubbles into the wastewater pond. One type of an oxygen diffuser for wastewater treatment process requires constant movement of the diffuser to different levels and positions within the wastewater pond and performs minimal mixing of the wastewater and oxygen. In addition, un-reacted air or oxygen bubbles make their way to the surface and do not become dissolved in the liquid. If oxygen is the source, then the oxygen that makes it to the surface of the wastewater pond is wasted as it vents to the air above the pond.
Mechanical aeration and mixing systems take on various forms, such as downdraft pumps, which force surface water to the bottom, updraft pumps, which produce a small fountain, and paddle wheels, which increase the surface area of the water. In addition, all such devices mix wastewater by moving large amounts of heavy water or hurling it into the air resulting in high energy consumption for these devices. Some such devices generate large amounts of odor and foam while agitating the wastewater and consume large amounts of electrical power resulting in high electricity cost for operation.
Therefore, it is readily apparent that there is a need for an economical apparatus and method for aeration of wastewater, sewage and industrial waste, or other liquids, such as fresh, salt and brackish water, and more particularly, a process for efficiently adding dissolved oxygen into such liquids while minimizing odor, foam and energy consumption.